1. Field of the Invention
The present invention relates to input devices for electronics and, more particularly, to a touch sensitive input panel or display with small form factor especially suited for use in cellular phones and personal digital assistants (PDAs), PC Tablets, as well as laptops, PCs, office equipment, medical equipment, or any other device that uses touch sensitive displays or panels.
2. Description of the Background
Touch screens have been deployed in many products in recent years.
There are several types of technology used in implementing touch sensitive screens that can detect the application of fingers and other passive objects.
For example, resistive pads comprise two conductive plates pressed together. The disadvantage of a resistive pad is that it requires a fixed amount of pressure by a finger in order to make the contact between the two conductive plates. This leads to user fatigue due to non-user optimized touch screen sensitivity. The resistive membrane will also wear out, initially resulting in further reduced clarity followed by dead spots.
Capacitive touchpads operate by measuring the capacitance of the passive object to ground, or by measuring the alteration of the transcapacitance between different sensors. An example of a capacitive touchpad is described in U.S. Pat. No. 5,495,077 to Miller. Capacitive pads are relatively expensive to manufacture, and can only detect objects with sufficient capacitance. Small objects, such as the end of a regular stylus or pen, do not have enough capacitance to ground or transcapacitance to be detected by a capacitive touchpad.
Surface acoustic wave devices operate by emitting sound along the surface of the pad and measuring the interaction of the passive object with the sound. These devices work well, but are generally much too expensive for general applications.
Finally, there are devices that use force sensors to measure the location and magnitude of the force exerted by the passive object on the touchpad. A force sensitive touchpad will sense force applied by any sort of passive object, regardless of the electrical conductivity or composition of the object. Such devices were originally described in U.S. Pat. No. 3,657,475 to Peronneau et al. and U.S. Pat. No. 4,121,049 to Roeber. These devices measure the forces transmitted by the touchpad to a fixed frame at multiple points e.g., at the corners of the pad. Roeber discloses a mathematical formula for deriving the position and magnitude of the force applied by a passive object from the forces measured at the multiple points.
For example, U.S. Pat. No. 4,511,760 to Garwin et al. issued Apr. 16, 1985 shows a force sensing data input device responding to the release of pressure force. The input surface is provided with a transparent faceplate mounted on force-sensing piezoelectric transducers. Preferably, four piezoelectric transducers are provided, one at each corner of a rectangular opening formed in the frame. To determine the point of application of force on the input surface, the outputs of the four transducers are first summed. To constitute a valid data entry attempt, the sum must exceed a first threshold while the user is pushing on the input surface. When the user releases his finger, a peak of the sum is detected, which is of opposite polarity from the polarity of the sum for the pushing direction. The individual outputs of the four sensors at the time that the peak of the sum occurs are used to calculate the point of application of the force.
United States Patent Application 20030085882 by Lu published May 8, 2003 shows a touch pad device having a support layer with a plurality of strain gauges in a matrix configuration. A touch layer is disposed on top of the strain gauge matrix, the touch layer being joined to the top of the strain gauge matrix. Sensor wires connect the strain gauges to a processor which is programmed with an algorithm to measure the location and pressure of simultaneous, multiple touches.
United States Patent Applications 20040108995 and 20040021643 both by Hoshino et al. show a display unit with touch panel mounted above a display via four differentially-mounted sensors. The pressure sensors detect force with which a pointing device such as a finger pushes the panel surface, in real time. The force P with which the pointing device such as a finger pushes the panel surface is found from the following equation irrespective of the pointing position: P=a+b+c+d−a0+b0+c0+d0, which equation detects dragging of a cursor.
United States Patent Application 20050156901 by Ma et al. issued Jul. 21, 2005 shows a touch screen display system with a display screen and overlying touch surface. An imaging system determines an angular position on the touch surface of the object coming in contact with the touch surface.
United States Patent Application 20060119589 by Rosenberg shows a haptic feedback feature for touchpads and other touch controls in which at least one actuator is coupled to the touch input device and outputs a force to provide a haptic sensation to the user contacting the touch surface. Output haptic sensations on the touch input device can include pulses, vibrations, and spatial textures. The claims require touch panel mounted on a suspension, and an actuator configured to output haptic feedback to the compliant suspension which amplifies the haptic feedback.
United States Patent Application 20060016272 by Chang published Jan. 26, 2006 shows a thin film touch pad with opposed sensor elements that generate an electrical signal that is proportional to both the applied pressure and the surface area at the location of the applied pressure. As a result of the complementary orientation and overlapping for these sensor elements, the first and second sensor elements generate an asymmetric pair of signals that uniquely define the applied pressure by position and magnitude.
U.S. Pat. No. 6,879,318 by Chan et al. issued Apr. 12, 2005 shows a touch screen mounting assembly for a liquid crystal display panel LCD including a bottom frame, a backlight panel seated in the frame and that has a plurality of pressure-sensitive transducers mounted thereon, a liquid crystal display panel, and a top frame for exerting pressure when mounted to the bottom frame such that a plurality of compressible springs biases the LCD panel towards the bottom frame when touched or contacted by a user. The claims require the bottom and top frame assembly with backlight panel mounted therein on springs, and an overlying LCD panel.
Despite the foregoing, a commercially viable force-based touch sensor for use with consumer equipment, such as computers, must be both inexpensive and precise. The precision required of such a device is the capability to sense both fingers and pens over a pressure range from about 1 gram to 300 grams or more, with a positional precision of 9 bits over this range. This precision level requires a sensor that can measure typical displacements of up to about 0.01-0.1 mm (mils) with a noise floor of 14 bits. When used in a smaller electronics device the sensor must also be thin, typically less than about 2 mm, although for some products it may be up to 20 mils maximum thickness, and should also be capable of modular assembly for more-or-less “snap-in” construction. In today's electronic industry the manufacturer of an electronic device utilizing a pressure sensitive touch sensitive display solution will look to their display supplier for a complete solution that is ready to be integrated into the overall product design. Despite the availability of the existing sensing technologies mentioned herein, the prior art has been unable to provide a low-cost sensor assembly having sufficient accuracy and form factor.
It would, therefore, be greatly advantageous to provide a force sensing technology which overcomes some of the deficiencies of the prior art.